Polymer nanoparticles have attracted increased attention over the past several years in a variety of fields including catalysis, combinatorial chemistry, protein supports, magnets, and photonic crystals. Similarly, vinyl aromatic (e.g. polystyrene) microparticles have been prepared for uses as a reference standard in the calibration of various instruments, in medical research and in medical diagnostic tests. Such polystyrene microparticles have been prepared by anionic dispersion polymerization and emulsion polymerization.
One benefit of using nanoparticles as an additive in other materials is that they can be discrete particles conducive to uniform dispersion throughout a host composition. For certain applications nanoparticles are preferably monodisperse in size and uniform in shape. However, controlling the size of nanoparticles during polymerization and the surface characteristics of such nanoparticles, or both, can be difficult. Accordingly, achieving better control over the surface composition of such polymer nanoparticles is also desirable.
Development of nanoparticles having a surface layer or shell that can include a variety of functional groups or heteroatomic monomers that would be compatible with a wide variety of matrix materials is desirable. However, the development of a process capable of reliably producing acceptable nanoparticles with a variety of functional groups or heteroatomic monomers has been a challenging endeavor. For example, the solubility of various monomers in traditional alkane solvents has made solution polymerization a difficult process by which to achieve nanoparticles having a tailored variety of shell layers. Emulsion synthesis requires the use of aqueous solutions to synthesize the nanoparticles and many functional monomers and initiators are not suitable to be used in aqueous solutions. In addition, emulsion synthesis also requires a large amount of surfactants, which may be undesirable for several reasons. Furthermore, functionalizing nanoparticles with certain functional groups can be difficult, if not impossible, because the functional group must be stable enough to survive the nanoparticle formation steps. In addition, post-nanoparticle formation functionalization may cause nanoparticles to bond together, leading to loss of their discrete nature.
Herein, a method is provided for synthesizing a core-shell nanoparticle that includes the following steps: providing a polymeric seed (in a solvent) that includes a mono-vinyl monomer cross-linked with a cross-linking agent to form the core of the nanoparticle, the core has an average diameter of about 5 nanometers to about 10,000 nanometers, and the core has polymer chains with living ends; adding a stabilizer to stabilize the seed and prevent the seed from precipitating out of solution; and grafting and/or polymerizing a shell species onto the living ends of the core to form the shell of the nanoparticle.
In addition, a method for making a rubber composition is provided. The method includes the steps of: making a core-shell nanoparticle(s) as described in the preceding paragraph and adding the core-shell nanoparticle(s) to a vulcanizable rubber matrix to form a rubber composition.
A method for making a tire is also provided. The method includes the steps of making core-shell nanoparticles as described above; adding the core-shell nanoparticles to a rubber composition; molding the rubber composition into a tire tread; and constructing a tire using the tire tread.
Furthermore, a core-shell nanoparticle is also provided. The core-shell nanoparticle includes a core formed from a polymeric seed that includes a mono-vinyl monomer cross-linked with a cross-linking agent, the core having an average diameter of about 5 nanometers to about 10,000 nanometers. A shell comprising a shell species is grafted and/or polymerized to the core, the shell being substantially uncrosslinked.
Herein throughout, unless specifically stated otherwise: “vinyl-substituted aromatic hydrocarbon” and “alkenylbenzene” are used interchangeably; and “rubber” refers to rubber compounds, including natural rubber, and synthetic elastomers including styrene-butadiene rubber and ethylene propylene rubber, which are known in the art. Furthermore, the terms “a” and “the,” as used herein, mean “one or more.”